Invited speaker: Andrea AlbertiAffiliation: Institut für Angewandte Physik, Universität BonnTitle: Discrete-time quantum machines using neutral atoms in optical latticesTime and room: 14:15, lecture hall HISKPAbstract: Neutral atoms trapped in optical lattices have been instrumental in the past years to advance our understanding of quantum phases of matter, for the determination of fundamental constants, and for numerous applications in quantum technology, ranging from quantum sensors and time-keeping, up to quantum simulations of complex many-body systems. Neutral atoms in optical lattices also provide a promising platform to store and process quantum information, where large ensembles of identical atoms can be prepared and manipulated with control at the single-particle and single-site level [1].

In this colloquium, I will present experiments in which the optical lattice potentials are made to depend on the electron spin state of caesium atoms in order to realize discrete-time quantum machines [2]. In a discrete-time quantum machine, the time evolution—instead of being determined by a static Hamiltonian—is governed by a series of discrete operations, which are rapidly applied in sequence. Using the extra degree of freedom provided by the spin, we can transport atoms in space along different spin-dependent quantum paths with subnanometer precision. In this way, we can achieve fast delocalization of matter waves on a time scale of 10 µs, which is two orders of magnitudes faster than the tunnelling time in a shallow optical lattice. Very recently, using optimal quantum control theory, we could speed up the delocalization process up to so-called quantum speed limit of our optical lattice system.

An example of a discrete-time quantum machine at the single particle level is provided by quantum walks: Depending on its spin state, the atom is moved, at regular time steps, either one site to the left or to the right, delocalizing it over multiple quantum paths. By “reprogramming” the operations defining one step of the quantum walk, we have simulated charged particles in external electric [3] and magnetic fields [4], and studied novel topological phases of periodically driven band insulators [5]. On a more fundamental level, relying on ideal negative measurements, we have tested the “quantum¬ness” of the walk, demonstrating a 6-σ violation of the Leggett-Garg inequality, which rules out any macro-realistic interpretation based on well-defined trajectories [6].

I will conclude with an outlook towards Hong-Ou-Mandel-like interference experiments, which enable the detection of quantum statistics using a pair of distant atoms [7]. Generalizations to a higher number of identical atoms hold the promise to construct an atom BosonSampling machine with a large number of indistinguishable particles, which can be scaled well above the 50-particle limit of classical simulations based on today’s supercomputers.

Invited speaker: Herwig OttAffiliation: Technische Universität KaiserslauternTitle: Rydberg Physics Meets Ultracold Quantum GasesTime and room: 17:15, lecture hall IAPAbstract: During the last two decades, ultracold quantum gases have become a valuable experimental platform for many-body physics, and a series of groundbreaking studies with bosonic and fermionic quantum gases has been carried out. At the same time, cooling and trapping of ultracold atoms has revolutionized the field of Rydberg physics, a discipline, which has its origin in atomic physics. Today, both research directions are closely linked to each other.

In my talk, I will show how the two formerly disjunct areas of physics can benefit from each other. In particular, I will show that so-called Rydberg molecules can be employed to tune the interaction in an ultracold quantum gas via an optical Feshbach resonance.

Invited speaker: Jakob ReichelAffiliation: Laboratoire Kastler Brossel, ENS ParisTitle: Creating Multiparticle Enganglement with Optical Fiber MicrocavitiesTime and room: 17 h c.t., lecture hall IAPAbstract: An exciting and fast-growing research field has emerged at the interface of fundamental physics and technology, where the nonclassical features of quantum mechanics are employed to engineer powerful, radically new functionalities. Multiparticle entanglement is a key resource in these quantum technologies. I will describe how high-finesse optical cavities can be used to produce and detect such entanglement in ensembles of ultracold atoms and other quantum emitters, and show examples from recent experiments in our group with fiber microcavities on atom chips. One application is quantum metrology, and I will show progress towards a spin-squeezed atomic clock on a chip.

Time and room: 16:00, lecture hall IAPAbstract: Hard computational problems may be solved by physics systems that can simulate them. Here we present a new a new system of coupled lasers in a modified degenerate cavity that is used to solve difficult computational tasks. The degenerate cavity possesses a huge number of degrees of freedom (>300 000 modes in our system), that can be coupled and controlled with direct access to both the x-space and k-space components of the lasing mode. Placing constraints on these components can be mapped to different computational minimization problems. Due to mode competition, the lasers select the mode with minimal loss to find the solution. We demonstrate this ability for simulating XY spin systems and finding their ground state, for phase retrieval, for imaging through scattering medium and more.

Time and room: 17:15, lecture hall IAPAbstract: The control of quantum states is essential both for fundamental investigations and for technological applications of quantum physics. In quantum few-body systems, decoherence arising from interaction with the environment hinders the realization of desired processes. In quantum many-body systems, complexity of their dynamics further makes state preparation via external manipulation highly non-trivial. An effective strategy to counter these effects is offered by quantum optimal control theory, exploiting quantum coherence to dynamically reach a desired goal with high accuracy even under limitations on resources such as time, bandwidth, and precision. In this talk I will:
- introduce the quantum optimal control method we developed to this aim, the CRAB (Chopped Random Basis) algorithm, which is to date the only method that allows to perform optimal control of quantum many-body systems;
- present experimental results obtained via its application to various physical systems, from quantum logical operations in solid-state quantum optics to quantum criticality in ultra-cold atoms, both in open-loop and in closed-loop feedback scenarios, with applications ranging from quantum interferometry with Bose-Einstein condensates on atom chips to magnetic field sensing in diamond NV centers and to the preparation of optical-lattice quantum registers for quantum simulation;
- use these examples to illustrate the quantum speed limit, i.e. the maximum speed achievable for a given quantum transformation, and describe related effects of nonlinearity due to inter-particle interactions and more in general to dynamical complexity;
- propose a way to characterise the latter in an information-theoretical fashion by the bandwidth of the optimized control pulses, as well as a conjecture about using this method for discrimination between different levels of complexity in quantum many-body systems.